Add QCEW CSV module, ECEC breakdown, response caching + typed errors

Item 1 — QCEW county/industry detail:
- new bls_client/qcew.py: thin client over the QCEW Open Data CSV service
  (area/industry/size slices; no API key, no quota), with filter_rows helper
- covers the county/industry detail the timeseries API can't reach

Item 2 — ECEC benefit breakdown:
- new series.ecec() builder from the authoritative cm.estimate decode table
- real helpers: wages, total benefits, paid leave, supplemental pay,
  health insurance, retirement & savings, legally required + ecec_dashboard()
- FIX: ecec_total_benefits was CMU1036... (education/health industries only);
  correct all-civilian total benefits is CMU1030000000000D

Item 3 — caching + typed errors:
- bls_client/cache.py FileCache (on-disk, TTL); BLSClient(cache=True),
  queries_used counter (cache hits don't spend quota)
- bls_client/errors.py: BLSError / BLSQuotaError / BLSRequestError; quota
  exhaustion now raises a clear BLSQuotaError instead of a generic RuntimeError

Tests: 119 offline (+24) incl. ECEC goldens, QCEW fixture parse, cache/quota
behavior; live smoke extended to ECEC + QCEW. Helper sweep: 96/96 live.
README: caching/QCEW usage, updated coverage + limitations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_query_helpers.py

@@ -18,7 +18,10 @@ GOLDEN = [
     (P.ppi_finished_goods,                         "WPUFD4"),   # back-compat alias
     (W.eci_wages,                                  "CIU1020000000000A"),
     (W.eci_total_compensation,                     "CIU1010000000000A"),
-    (W.ecec_total_benefits,                        "CMU1036000000000D"),
+    (W.ecec_total_compensation,                    "CMU1010000000000D"),
+    (W.ecec_total_benefits,                        "CMU1030000000000D"),
+    (W.ecec_health_insurance,                      "CMU1150000000000D"),
+    (W.ecec_retirement,                            "CMU1180000000000D"),
     (PR.business_output_per_hour,                  "PRS84006092"),
 ]